A wireless network is a flexible data communications system, which uses wireless media such as radio frequency technology to transmit and receive data over the air, minimizing the need for wired connections.
Wireless networks use electromagnetic waves such as e.g. radio signals or infrared signals, to communicate information from one point to another without relying on any physical connection. One type of electromagnetic waves, radio waves, are often referred to as radio carriers because they simply perform the function of delivering energy from a local transmitter to a remote receiver. The data being transmitted is superimposed on the electromagnetic wave, in the example given a radio carrier, so that it can be accurately extracted at the receiving end. Once data is superimposed (modulated) onto the radio carrier, the radio signal occupies more than a single frequency, since the frequency or bit rate of the modulating information adds to the carrier. Multiple radio carriers can exist in the same space at the same time without interfering with each other if the radio waves are transmitted on different radio frequencies. To extract data, a radio receiver tunes in one radio frequency while rejecting all other frequencies. The modulated signal thus received is then demodulated and the data is extracted from the signal.
Wireless networks are highly desirable communication networks because they do not suffer from the cost and labour of installing cables (eliminating the need to pull cables through ceilings and walls), and they permit greater flexibility in locating and optionally moving communication equipment (the network can be extended to places that cannot be wired).
One particular field for usage of wireless networks is sensor applications. The requirements of wireless sensor applications are straightforward: network coverage of the sensor environment must be absolute, the installation process should be plug-and-play and the reliability must be extremely high. The multi-hop or mesh network concept as shortly explained below, provides an answer to all three challenges.
Multi-hop networks comprise a plurality of nodes organised into a network of wireless connection paths between the nodes. Multi-hopping is a way of routing data, voice and instructions between the nodes in the network. It allows for continuous connections and reconfiguration around blocked paths by “hopping” from node to node until a connection can be established.
In a multi-hop network, there is typically one special node, called a network co-ordinator that starts the network. All other nodes connect to the network through this co-ordinator or through another node that was connected to the network before.
In multi-hop wireless networks intermediate nodes act as repeaters and/or routers for other nodes. Hence messages hop from one node to another, thereby bridging longer distances than could be spanned by a single wireless link between two nodes. In order for a first node to be able to repeat/route messages from a second node, the first node is first set in a receiving state. After reception of the message from the second node, the first node is set in a transmitting state, for repeating or routing the message further to a third node, which in turn is set in a receiving state.
Multi-hop networking provides an answer to the challenge of network coverage put forward by wireless sensor applications: as nodes are dispersed over a limited location, e.g. a building, and every node is able to communicate to its neighbour nodes that are within electromagnetic wave reception distance, the full environment is actually enveloped in a cloud of electromagnetic waves, e.g. radio waves. State of the art multi-hop network technology may be self-forming and self-healing. This means that the network will set itself up without manual intervention. New nodes will associate automatically to the network. When a node is removed or a link between any two nodes is disturbed, then the network automatically reconfigures on-the-fly to re-establish reliable communication (self-healing property). This means that the network can still operate even when a node breaks down or a connection goes bad. As a result, a very reliable network is formed.
The traditional answer to low-power requirements is to lower the duty cycle of the application: nodes wake-up every once in a while to transmit information and go in deep sleep (low-power) otherwise. This solution is also applied for achieving low-power operation of nodes in a multi-hop wireless network. When in a sleeping state, a node is not able to receive messages from other nodes and neither is it able to perform its repeater/router function.
As can be easily seen by a person skilled in the art, the requirements of “multi-hop capable” and “low-power” are contradictory per se. In a multi-hop network one relies on neighbour low-power nodes to forward messages. But neighbour nodes in sleeping-mode will never hear the message to be forwarded. Therefore, in order to allow for low-power operation, yet at the same time allow multi-hop network operation, some synchronization in the sleep and wake-up cycles of nodes is needed, at least between neighbouring nodes.
Neither basic WIFI nor Bluetooth use the concept of multi-hop networking. Basic WIFI relies on the base-station concept: a base-station serves a local area of approx. 15 m in diameter. Spots further away from the base-station have limited or no radio coverage. Advanced Win base stations, however, can also be used as a wireless router. Mostly they are setup up manually, possibly with help of some installation wizard software to make the setup a bit easier. Multi-hop networks on WIFI is a big research domain, they are often called ‘MANET’s (metropolitan area networks or mobile ad-hoc networks). Such MANET is a kind of wireless ad-hoc network, and is a self-configuring network of mobile routers (and associated hosts) connected by wireless links, the union of which forms an arbitrary topology. The routers are free to move randomly and organise themselves arbitrarily; thus, the network's wireless topology may change rapidly and unpredictably. Bluetooth is based on the scatternet principle, where nodes can associate with one or more neighbour nodes, but have no ability to route messages between two neighbours as in a mesh network.
From a power saving standpoint, the duty cycle of each node in a multihop network should be as low as possible. However, as nodes perform a co-operative task, they need to co-ordinate their sleep/wake-up times. Hence, a sleep/wake-up scheduling algorithm is required. Ideally, it should allow neighbouring nodes to be active at the same time, thus making packet exchange feasible even when nodes operate with a low duty cycle (i.e. they sleep for most of the time). “Low duty cycle” reduces the energy consumption of nodes significantly as it keeps nodes active mainly only when there is network activity. There are presently three types of sleep/wake up algorithms: scheduled rendezvous, asynchronous and on-demand.
Scheduled rendezvous schemes require that all neighbouring nodes wake up at the same time. Typically, nodes wake up periodically to check for potential communication. Then, they return to sleep until the next rendezvous time. A major advantage of such schemes is that when a node is awake it is guaranteed that all its neighbours are awake as well. This allows sending broadcast messages to all neighbours. However such schemes require clock synchronisation. Different scheduled rendezvous protocols differ in the way network nodes sleep and wake up during their lifetime. One way is using a Fully Synchronised Scheme. In this scheme all nodes in the network wake up at the same time periodically and remain active for a fixed time. Then, they return to sleep until the next wake-up point. A further improvement can be achieved by allowing nodes to switch off their radio when no activity is detected after a timeout value. One disadvantage of the fully synchronised scheme is that nodes try to transmit simultaneously in the active time, thus causing a large number of collisions. One technique to avoid this is to use staggered wake-up times.
Asynchronous schemes avoid the tight synchronisation among network nodes required by scheduled rendezvous schemes. They allow each node to wake up independently of the others but guarantee that neighbours always have overlapping active periods within a specified number of cycles.
On-demand schemes are based on the idea that a node should awaken only when it has to receive a packet from a neighbouring node. This minimises the energy consumption. In such a network, nodes are in a monitoring or listening state for most of the time. As soon as an event is detected, nodes transit to the active or transfer state. On-demand sleep/wake-up schemes are aimed at reducing energy consumption in the monitoring state while ensuring a limited latency for transitioning in the transfer state. The implementation of such schemes typically requires two different channels: a data channel for normal data communication, and a wake-up channel for awaking nodes when needed. All present proposals rely on two different radios, e.g. a low power wake-up radio and a main radio for traffic. A drawback is the additional cost for the second radio.
EP-1657852 relates to an energy efficient mechanism for establishing and maintaining a communication between nodes in a wireless communication system. A destination node listens to a communication channel periodically. For requesting services from the destination node, a wake-up signal is transmitted from a source node to the destination node via the communication channel. The wake-up signal comprises a repetition of a signal block, each signal block comprising a preamble and a data packet. The wake-up signal duration and content is adapted depending on the system operation context to reduce either idle times, power consumption, latency or network blocking. However, it is a disadvantage of this system that it still has a high power consumption and a risk of data collision.
Schurgers C. et al. describe in “Topology Management for Sensor Networks: Exploiting Latency and Density”, Mobihoc 2002, Proceedings of the 3rd ACM International Symposium on Mobile ad hoc networking and computing, Lausanne, Switzerland, 9 Jun. 2002, a wireless sensor network comprising a plurality of nodes. The communication between two nodes is done at two different frequencies: f1 for the wake-up signal, f2 for the data transmission. According to this document, when a node wants to wake up one of its neighbours, it starts sending beacon packets until it receives a response from the target node.